I Field of the Invention
The present invention relates to protective devices for suppressing short duration, large current impulses, such as lightning strikes, which may occur along coaxial cables or other HF, VHF or UHF transmission lines. More particularly, the invention relates to the use of a gas discharge tube in combination with a connector for being inserted in series with the transmission line.
II Description of the Prior Art
The use of vacuum tubes in prior radio frequency transmitting and receiving equipment made them somewhat tolerant to nearby lightning strikes since the breakdown voltage of the tubes was relatively high and the tubes would typically not be damaged unless there was a direct lightning strike on the antenna or the feedline. On the other hand, recent advances in solid state design technology have allowed transistors to replace tubes in most applications. The problems of surge protection or lightning strikes for transistorized receivers or transmitters is especially troublesome in view of the low breakdown voltages for typical solid state devices. Once this low breakdown voltage has been exceeded, the solid state device is no longer operative and must be replaced.
Solid state devices of this type are presently being widely utilized in television receivers, television receiving convertors, cable television distribution and amplification systems and other similar VHF and UHF radio frequency systems. The proliferation of solid state devices in systems such as these substantially increases the probability of a large number of complex and expensive electronic devices being destroyed by one well-placed lightning strike. The cost of the lightning or surge protection has become more economical in view of the large cost of repairing this equipment. This cost factor becomes even more economical when the lightning or surge protection device can withstand multiple lightning strikes of reasonable intensity without the necessity of replacing the protection device or without destruction of any of the equipment attached thereto. However, these economies of lightning protection are not acceptable if the performance of the system in which the lightning protection device is used is degraded by the insertion of the protection device. Transmitting systems are of the greatest interest in this regard since the insertion loss and VSWR along the transmission line are somewhat critical at VHF and UHF frequencies.
The prior art has many examples of electro-magnetic impulse protection devices for radio frequency transmission lines. The earliest devices employs a grounding strap which merely grounded both sections of the transmission line in order to reduce the likelihood of static electricity buildup and the concomitant likelihood of a lightning strike. This solution is obviously unacceptable when continuous transmission of radio frequency energy is required.
Later impulse protection systems employed air gaps in order to allow the lightning or impulse signal to arc across the gap and thereby travel to ground. One example of a device of this type employing air gaps is described by Cushman in U.S. Pat. No. 2,922,913. This device is presently being marketed under the trademark BLITZ-BUG. Devices of this type suffer from several different problems. First, since the device exists in the ambient atmosphere, any arc drawn from one of the spark gaps will cause severe vaporizaton or oxidation of the gap electrodes. This degredation of the electrodes will substantially increase the gap firing voltage above the level tolerated by solid state devices. In the extreme, the oxidation or vaporization of the electrodes can render the device useless after one or two lightning strikes. Since there is no external indication of the occurrence of such a lightning strike or the uselessness of the spark gaps internal to the device, the system is left completely unprotected while the device outwardly appears to be operative. Frequent disassembly and inspection of the gaps are usually required. Secondly, the large air gaps utilized in devices of this type are not suitable for transistorized equipment. Breakdown voltages of 1500 to 2000 volts are typically required in order to cause an arc to occur between the electrode elements across the air gap. Transistors often will be destroyed by voltages well below this level.
Nelson, in U.S. Pat. No. 3,274,447, discloses a coaxial connector of the type employing an internal gap for allowing the impulse to arc to ground potential. Devices of this type, while more suitable for insertion into coaxial transmission lines, suffer from the same basic oxidation and vaporization problems as described with regard to U.S. Pat. No. 2,922,913.
Other inventors have concentrated on combining protection for radio frequency transmission lines with protection for AC electrical supply protection. Simokat in U.S. Pat. No. 4,050,092, assigned to the TII Corporation of Lindenhurst, N.Y., is an example of a gas-filled tube being utilized to shunt the electrical energy from a primary electrical conductor to ground in order to protect the sensitive electronic solid state devices coupled to the transmission line. This particular device also protects the AC power lines feeding the receiver or transmitter from the same electrical surge. Devices of this type are not suitable for use at high frequencies, because contrary to the teachings of Simokat, no precautions have been taken to assure proper impedence matching and to minimize the insertion loss of the device in the VHF or the UHF transmission lines. Also, the device as described by Simokat is primarily related to receiving applications and would not be suitable for applications involving transmission of radio frequency power. Furthermore, the inherent design of the device as disclosed by Simokat is not suitable for impedence matching for proper operation at UHF frequencies (as used herein UHF frequencies will refer to the frequencies above 400 MHz and below 3,000 MHz).
The Simokat gas-filled tube impulse protection device is widely used on low frequency transmission lines such as power lines, telephone lines, low speed data lines, etc. However, the use of these gas-filled tubes has not been generally successful on radio frequency transmission lines without a substantial degredation of the characteristic impedence of the signal transmission line. This impedence anomaly causes the occurrence of standing waves (VSWR), signal losses, and group phase delays which are highly undesirable and detrimental to the proper functioning of most communications systems.
Martzloff, in U.S. Pat. No. 3,863,111 assigned to the General Electric Company, attacks the surge protection problem by providing a coaxial-type connector which employs a polycrystalline varistor for surge protection. A spring is provided to compress the varistor into electrical contact with ground potential. The spring is designed to form a resonant circuit in conjunction with the conductors within the connector. This spring acts as an inductor which is a low impedance to the relative low frequencies of the impulse, but is a relatively high impedance at higher frequencies. Designs of this type typically are suitable only for use in the HF or VHF region (below 50-100 MHz). The device is typically not usable at frequencies below the self resonant frequency of the coil, and the multiple higher resonant frequencies of the coil and various internal capacitances indicate that, at least at the higher frequencies the insertion loss will substantially increase and the attentuation curve as a function of frequency will be extremely uneven. The reactance of the coil and its related circuit will cause a relatively high VSWR to occur on the line and at every series resonant point. These points occur due to stray capacitances. The insertion losses of devices of this type can be substantial at VHF frequencies. Furthermore, the power handling capability of varistors of this type are highly suspect. Devices of this type are usually used only for receiving applications and are not suitable for high power transmitter applications.
Winters, in U.S. Pat. No. 3,777,219, discloses a coaxial connector device which defines an internal cavity. A plurality of semi-conductor wafers employing silicon junction avalanche-type diodes are carried within the cavity. The occurrence of a large voltage impulse along the center conductor of the device will be shorted to ground (the outside braid of the coaxial connector cable) when the impulse voltage exceeds the threshold voltage of the silicon junction avalanche diodes. Avalanche diodes of this type are not well-suited for high power transmission applications because no effort has been made to make the apparent impedance of the unit completely transparent to all RF energies by making it as an integral section of transmission line. Furthermore, the power handling capabilities of the avalanche diodes are somewhat limited, with an 8 microsecond rise and a 20 microsecond decay time being typical. Devices of this type are typically limited to receive only applications and therefore impedance matching at the higher frequencies is not as critical.
The capacitive effects of the diodes limit the design of this protection device to high frequency spectrum applications. In order to use it for transmission of R.F. energy, the number of diodes must be increased in the series configuration in order to increase the series avalanch voltage. This reduces the current handling capabilities of the device since each diode has a substantial series resistance value. As more diodes are added in series, the total "on" resistance value increases. If the breakdown voltage of each individual diode is increased to handle more power, the size of the diode must also increase as the junction area increases. This also causes an increase in the "off" capacitance for each diode, which will limit the high frequency usage of the device. The diode has a very fast turn-on time, about 10 better than a gas tube, but it has smaller current handling capabilities and power dissipation factors.
McNatt, in U.S. Pat. No. 2,886,744, discloses a coaxial connector device which employs a series connected fuse in the primary circuit conductor. A choke or discreet inductor is coupled from the primary or center circuit conductor to the outside shield conductor. The inventor indicates that this choke will typically limit the use of this device to frequencies in the 25-30 MHz range, which is at the very lowest edge of the VHF frequency bands. A device of this type would not be suitable for use at higher frequencies (such as above 50-100 megacycles) and would not be suitable for use with high powered transmitters.
Various other lightning or surge protection devices are described by Fuller in U.S. Pat. No. 2,896,128, Braumm, in U.S. Pat. No. 3,450,923, Jackson in U.S. Pat. No. 1,194,195, Pacent in U.S. Pat. No. 1,527,525, Finkel in U.S. Pat. No. 2,654,857, Grassnick in U.S. Pat. No. 2,237,426, Epstein in U.S. Pat. No. 2,277,216, Boylan in U.S. Pat. No. 2,957,110, Klostermann in U.S. Pat. No. 2,666,908 and Craddock in U.S. Pat. No. 1,892,567. Various other lightning protection and surge protection devices are disclosed by Clark in U.S. Pat. No. 3,934,175, and Brown in U.S. Pat. No. 3,840,781.
Gilberts in U.S. Pat. No. 4,158,869, discloses the use of a gas discharge tube in a device for protecting telephone lines from electrical impulses or lightning strikes. Lundsgaard, in U.S. Pat. No. 4,142,220, also discloses the use of a gas discharge tube for protecting telephone lines. The present inventor has examined both of these references and does not believe that either of the references is suitable for use at UHF frequencies where impedence matching and insertion losses are of critical importance. Neither of these devices teach the use of an impedence matching technique whereby the lumped inductances and capacitances, when taken together, represent the same characteristic impedance of the connector and surge protector as compared to the coaxial feed lines.
In contrast to the prior art, the present invention relates to a connector of the type which may be inserted into a length of coaxial radio frequency cable, or other HF, VHF or UHF transmission line, for controlling and dissipating the surge energy (such as lightning) traveling from the antenna side toward the receiver/transmitter side, while not presenting a high VSWR or insertion loss when viewed from the transmitter end toward the antenna end of the line. The capacitance of the gas discharge tube used in the circuit, and other stray or distributed capacitances, are carefully balanced with distributed inductive reactance so that the characteristic impedance of the connector, when viewed as a lump element circuit, will correspond to the to the characteristic impedance of the transmission line. Thus, the connector will be transparent to the transmitted RF signal, but will be effective in dissipating or shunting the electrical impulse traveling down the line.